Ink composition, process for producing same, and ink-jet ink set and ink-jet printing system both including said ink composition

ABSTRACT

An ink composition according to the present invention contains a polymerizable compound and a photopolymerization initiator, wherein an amount of organic sulfonic acid measured at a temperature of 25° C. by using a water extraction method is 50 ppm or less, and an amount of water measured by a Karl Fischer method is 0.50 mass % or less with respect to a total mass of the ink composition. Also, an inkjet ink set according to the present invention includes the above-described ink composition of the present invention. Also, an ink-jet printing system according to the present invention uses the above-described ink composition of the present invention and an inkjet recording apparatus, and the inkjet recording apparatus includes an ink heating portion and an ink filter.

TECHNICAL FIELD

The present invention relates to an ink composition for use in anink-jet printer, a process for producing the ink composition, and anink-jet ink set and an ink-jet printing system both including the inkcomposition.

BACKGROUND ART

As inks applied to ink-jet technology, water-based inks composed mainlyof water as a solvent and oil-based inks composed mainly of an organicsolvent are used, but attention is now given to a solvent-free activeenergy radiation curable ink-jet ink that is cured through irradiationwith active energy rays (for example, ultraviolet rays) in order tosuppress ink bleeding on an image.

This type of active energy radiation curable ink-jet ink does notcontain a solvent, and thus it is unnecessary to cause the solvent topermeate into a recording medium, and the ink can be cured in a veryshort time. For this reason, a high level of printing quality with lessink bleeding can be obtained irrespective of the type of recordingmedium.

On the other hand, the active energy radiation curable ink-jet ink isproblematic in that precipitates are likely to be formed in the ink andcause clogging in the ink-jet head of an ink-jet printer when the ink isdischarged. The reason is as follows. A polymerizable compound that iscontained in the active energy radiation curable ink-jet ink is producedby using a catalyst such as an organic sulfonic acid, and thus normally,the catalyst remains in the polymerizable compound. When the ink issubjected to long-term storage, the catalyst leaches into the ink andreacts with other components, and the reaction product is precipitatedin the ink-jet head and causes clogging.

In order to solve the problem described above, Patent Document 1proposes a method in which a certain amount of water is contained in anink that contains: a photopolymerizable compound containing an acidcatalyst; and a photopolymerization initiator. Patent Document 1 statesthat with this method, it is possible to prevent the formation ofprecipitates from the ink.

PRIOR ART DOCUMENTS Patent Document

-   Patent Document 1: JP 2011-213801A

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, a printed matter formed by using the ink that contains water isproblematic in terms of printing performance such as the occurrence ofink bleeding on the image. For this reason, in the method proposed inPatent Document 1, it is necessary to eventually remove water by anymeans, which requires an apparatus for removing water, resulting in acomplex printing system.

The present invention has been made in order to solve the problemsdescribed above, and provides an ink composition that does not cause aprecipitate and has a high level of dischargeability a process forproducing the ink composition, and an ink-jet ink set and an ink-jetprinting system both including the ink composition.

Means for Solving Problem

The ink composition according to the present invention is an inkcomposition containing a polymerizable compound and aphotopolymerization initiator, wherein an amount of organic sulfonicacid measured at a temperature of 25° C. by using a water extractionmethod is 50 ppm or less, and an amount of water measured by a KarlFischer method is 0.50 mass % or less with respect to a total mass ofthe ink composition.

Also, a process for producing an ink composition according to thepresent invention includes the steps of preparing an ink compositionprecursor containing a polymerizable compound and a photopolymerizationinitiator; adding an alkali metal salt to the ink composition precursorand stirring the ink composition precursor at a temperature of 40° C. ormore and 70° C. or less for a period of 10 minutes or more and 120minutes or less; holding and aging the stirred ink composition precursorat a temperature of −20° C. or more and 35° C. or less; and filtratingthe aged ink composition precursor to produce an ink composition.

Also, another process for producing an ink composition according to thepresent invention includes the steps of preparing an ink compositionprecursor containing a polymerizable compound and a photopolymerizationinitiator; stirring the ink composition precursor at a temperature of40° C. or more and 70° C. or less for a period of 10 minutes or more and120 minutes or less; holding and aging the stirred ink compositionprecursor at a temperature of −20° C. or more and 35° C. or less; andfiltrating the aged ink composition precursor to produce an inkcomposition, wherein at least one of the polymerizable compound and thephotopolymerization initiator contains alkali metal ions.

An ink-jet ink set according to the present invention includes theabove-described ink composition according to the present invention.

The ink-jet printing system according to the present invention is anink-jet printing system that uses the above-described ink compositionaccording to the present invention and an ink-jet recording apparatus,wherein the ink-jet recording apparatus includes an ink heating portionand an ink filter.

Effects of the Invention

According to the present invention, it is possible to provide an inkcomposition that does not cause a precipitate and has a high level ofdischarpability, a process for producing the ink composition, and anink-jet ink set and an ink-jet printing system both including the inkcomposition.

Description of the Invention Embodiment 1

First, an ink composition according to the present invention will bedescribed. The ink composition according to the present inventioncontains a polymerizable compound and a photopolymerization initiator,and the amount of organic sulfonic acid measured at a temperature of 25°C. by using a water extraction method is 50 ppm or less, and the amountof water measured by using a Karl Fischer method is 0.50 mass % or lessof the total mass of the ink composition.

Because the amount of organic sulfonic acid measured at a temperature of25° C. by using a water extraction method is 50 ppm or less, the inkcomposition according to the present invention does not cause aprecipitate and has a high level of dischargeability. Also, because theamount of water measured by a Karl Fischer method is 0.50 mass % or lessof the total mass of the ink composition, in the ink compositionaccording to the present invention, the reaction sites where a reactionbetween alkali metal ions contained in the material and the organicsulfonic acid takes place are reduced, and it is therefore possible tosuppress the formation of a reaction product in the ink.

Hereinafter, the ink composition according to the present invention willbe described in detail.

Amount of Organic Sulfonic Acid

The amount of organic sulfonic acid in the ink composition according tothe present invention refers to a value measured at a temperature of 25°C. by using a water extraction method. The organic sulfonic acid iscontained in the polymerizable compound that is an ink component and isa residue of the organic sulfonic acid used as a catalyst whensynthesizing the polymerizable compound. In the ink compositionaccording to the present invention, the amount of organic sulfonic acidmeasured at a temperature of 25° C. by using a water extraction methodis preferably 50 ppm or less, more preferably 29 ppm or less, and evenmore preferably 7 ppm or less. The lower limit value for the amount ofthe organic sulfonic acid is ideally 0 ppm, but it is difficult tocompletely remove the organic sulfonic acid, and thus the lower limitvalue is about 1 ppm at most.

The water extraction method is a method in which the amount of organicsulfonic acid in a measurement solution produced by mixing the inkcomposition with water is measured by chromatography This method is usedbecause it is difficult to directly measure the amount of organicsulfonic acid in the ink composition. As the chromatography, forexample, liquid chromatography ion chromatography-, or the like can beused.

The organic sulfonic acid serving as a catalyst that is used whensynthesizing the polymerizable compound is normally at least oneselected from the group consisting of methane sulfonic acid, ethanesulfonic acid, trifluoromethane sulfonic acid, benzene sulfonic acid,dodecylbenzene sulfonic acid, toluene sulfonic acid, and naphthalenesulfonic acid.

Amount of Water

The amount of water in the ink composition according to the presentinvention refers to a value measured by a Karl Fischer method. The wateris unavoidably introduced during the production of the ink and thus isnot added intentionally. In the ink composition according to the presentinvention, the amount of water measured by a Karl Fischer method ispreferably 0.50 mass % or less of the total mass of the ink composition,more preferably 0.20 mass % or less, and even more preferably 0.03 mass% or less. The lower limit value for the amount of water is ideally 0mass %, but it is difficult to achieve 0 mass % in a normal productionenvironment, and thus the lower limit value is about 0.01 mass % atmost. An amount of water exceeding 0.50 mass % of the total mass of theink composition according to the present invention is not preferablebecause if there is water in such an amount, the water serves as areaction site where a reaction between the alkali metal ions in thematerial and the organic sulfonic acid takes place to produce a reactionproduct in the ink.

Polymerizable Compound

As the polymerizable compound, it is possible to use a monofunctionalmonomer having one ethylenic double bond within a molecule or amultifunctional monomer having a plurality of ethylenic double bondswithin a molecule that has a property of being cured by energy rays.

The polymerizable compound preferably contains an amine-modifiedpolymerizable compound. The amine-modified polymerizable compound isconsidered to suppress polymerization inhibition caused by oxygen in theair, and can improve the curing rate at the time of irradiation withultraviolet rays, in particular, at the time of irradiation withlow-energy ultraviolet rays using a light emitting diode (LED).

The amine-modified polymerizable compound is preferably a polymerizablecompound having at least one amino group (a primary, secondary ortertiary amine skeleton) within a molecule. Examples of thepolymerizable compound include amino(meth)acrylate, amine-modifiedpolyether(meth)acrylate, amine-modified polyester(meth)acrylate,amine-modified epoxy(meth)acrylate, amine-modifiedurethane(meth)acrylate, and the like. They may be used singly or in acombination of two or more. There is no particular limitation on theamount of the amine-modified polymerizable compound, but the amount ofthe amine-modified polymerizable compound is preferably 1 mass % or moreand 30 mass % or less, and more preferably 1 mass % or more and 11 mass% or less with respect to the total mass of the ink composition.

It is desirable that the amine-modified polymerizable compound has, inthe form of a cured product thereof, a glass transition temperature ofpreferably 25° C. or less, and more preferably 10° C. or less, in termsof adhesion. The glass transition temperature can be measured byirradiating, with ultraviolet rays having an energy with an integratedlight quantity of 1,000 mJ/cm², a mixture of the amine-modifiedpolymerizable compound and 1-hydroxy-cyclohexyl-phenylketone(1,2-α-hydroxyalkylphenone-based initiator) that serves as an initiator(with the mass ratio of the polymerizable compound to the initiatorbeing set to 97:3) so as to form a polymerized product, and subjectingthe polymerized product to measurement using a differential thermalanalysis apparatus (TG-DTA (2000S), which is a product available fromMac Science Corporation Ltd.).

It is desirable that the amine-modified polymerizable compound has amass average molecular weight of preferably 100 or more, and morepreferably 500 or more in terms of imparting flexibility to the inkcured product. Also, it is desirable that the amine-modifiedpolymerizable compound has a mass average molecular weight of preferably2000 or less, and more preferably 1500 or less in terms of lowering theink viscosity As used herein, the mass average molecular weight refersto a polystyrene-equivalent molecular weight measured by subjecting onlyan oligomer to gel permeation chromatography (GPC) (usingtetrahydrofuran as a solvent).

Specific examples of the amine-modified polymerizable compound include:EBECRYL80, EBECRYL81, and EBECRYL7100, which are products available fromDaicel-Allnex, Ltd.; CN371, CN550, and CN551, which are productsavailable from Sartomer, Co.; Laromer P094F, which is a productavailable from BASF SE; and the like.

As polymerizable compounds other than the amine-modified polymerizablecompound, the following can be used.

Specific examples of the monofunctional monomer having one ethylenicdouble bond within a molecule include amyl(meth)acrylate,isoamyl(meth)acrylate, octyl(meth)acrylate, isooctyl(meth)acrylate,decyl(meth)acrylate, isodecyl(meth)acrylate, lauryl(meth)acrylate,isomyristyl(meth)acrylate, stearyl(meth)acrylate,isostearyl(meth)acrylate, 2-ethylhexyl-diglycol(meth)acrylate,2-(meth)aaryloyloxyethyl hexahydrophthalate, neopentylglycol(meth)acrylate benzoic acid ester, butoxyethyl(meth)acrylate,ethoxy-diethylene glycol(meth)acrylate, methoxy-triethyleneglycol(meth)acrylate, methoxy-polyethylene glycol(meth)acrylate,methoxydipropylene glycol(meth)acrylate, phenoxyethyl(meth)acrylate,phenoxy-polyethylene glycol(meth)acrylate, nonyl phenol ethylene oxideadduct(meth)acrylate, tetrahydrofurfinyl(meth)acrylate,isobornyl(meth)acrylate, 2-hydroxyethyl(meth)acrylate,2-hydroxypropyl(meth)acrylate, 2-hydroxy butyl(meth)acrylate,2-hydroxy-3-phenoxypropyl(meth)acrylate, 2-(meth)acryloyloxyethyl-succinate, 2-(meth)acryloyloxyethyl-phthalate,2-(meth)acryloyloxyethyl-2-hydroxyethyl-phthalate, (meth)acrylatemonomers obtained by imparting a functional group such as phosphorus orfluorine thereto, and the like. They may be used singly or incombination.

Specific examples of the multifunctional monomer having two ethylenicdouble bonds within a molecule include hydroxypivalic neopentyl glycoldi(meth)acrylate, polytetramethylene glycol di(meth)acrylate,trimethylolpropane(meth)acrylate benzoic acid ester, diethylene glycoldi(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethyleneglycol di(meth)acrylate, polyethylene glycol (200) di(meth)acrylate,polyethylene glycol (400) di(meth)acrylate, polyethylene glycol (600)di(meth)acrylate, polyethylene glycol (1000) di(meth)acrylate,dipropylene glycol di(meth)acrylate, tripropylene glycoldi(meth)acrylate, polypropylene glycol (400) di(meth)acrylate,polypropylene glycol (700) di(meth)acrylate, neopentyl glycoldi(meth)acrylate, 1,3-butanediol di(meth)acrylate, 1,4-butanedioldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanedioldi(meth)acrylate, neopentyl glycol di(meth)acrylate,dimethylol-tricyclodecane di(meth)acrylate, bisphenol A ethylene oxideadduct di(meth)acrylate, bisphenol A propylene oxide adductdi(meth)acrylate, and the like. They may be used singly or incombination.

Specific examples of the multifunctional monomer having three ethylenicdouble bonds within a molecule include trimethylolpropanetri(meth)acrylate, pentaerythritol tri(meth)acrylate, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate, glyceryl tri(meth)acrylate, andtheir ethylene oxide-modified products, propylene oxide-modifiedproducts, and caprolactone-modified products, and the like. They may beused singly or in combination.

Specific examples of the multifunctional monomer having four ethylenicdouble bonds within a molecule include ditrimethylolpropanetetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, and theirethylene oxide-modified products, propylene oxide-modified products, andcaprolactone-modified products, and the like. They may be used singly orin combination.

Specific examples of the multifunctional monomer having five ethylenicdouble bonds within a molecule include dipentaerythritolhydroxypenta(meth)acrylate, and its ethylene oxide-modified products, propyleneoxide-modified products, and caprolactone-modified products, and thelike. They may be used singly or in combination.

Specific examples of the multifunctional monomer having six ethylenicdouble bonds within a molecule include dipentaerythritolhexa(meth)acrylate, and its ethylene oxide-modified products, propyleneoxide-modified products, and caprolactone-modified products, and thelike. They may be used singly or in combination.

The ink composition may further contain an oligomer or a prepolymer as apolymerizable compound.

There is no particular limitation on the amount of the polymerizablecompound in the ink composition, but the amount of the polymerizablecompound is preferably 55 to 98 mass % with respect to the total mass ofthe ink composition. When the amount of the polymerizable compound iswithin the above range, it is possible to improve the curability andadhesion of the ink.

Photopolymerization Initiator

As the photopolymerization initiator, it is preferable to use aphotopolymerization initiator that contains at least one compoundselected from the group consisting of an acylphosphine oxide compound,an α-amino alkyl phenone compound, and a thioxanthone compound that caninitiate a polymerization at low energy In particular, it is morepreferable to use an acylphosphine oxide compound, or a mixture of anα-amino alkyl phenone compound and a thioxanthone compound.

Specific examples of the acylphosphine oxide compound include2,4,6-trimethyl benzoyl diphenyl phosphine oxide, 2,6-dimethoxy benzoyldiphenyl phosphine oxide, 2,6-dichloro benzoyl diphenyl phosphine oxide,2,3,5,6-tetramethyl benzoyl diphenyl phosphine oxide, 2,6-dimethylbenzoyl diphenyl phosphine oxide, 4-methyl benzoyl diphenyl phosphineoxide, 4-ethylbenzoyl diphenyl phosphine oxide, 4-isopropylbenzoyldiphenyl phosphine oxide, 1-methyl cyclohexanoyl benzoyl diphenylphosphine oxide, bis(2,4,6-trimethyl benzoyl)-phenyl phosphine oxide,2,4,6-trimethyl benzoyl phenyl phosphinic acid methyl ester,2,4,6-trimethyl benzoyl phenyl phosphinic acid isopropyl ester,bis(2,6-dimethoxy benzoyl)-2,4,4-trimethyl pentyl phosphine oxide, andthe like. They may be used singly or in combination. An example of theacylphosphine oxide compound that is available on the market is DAROCURETPO available from Ciba AG.

Specific examples of the α-amino alkyl phenone compound include2-methyl-1-[4-(methyl thio)phenyl]-2-morpholinopropan-1-one,2-benzil-2-dimethylamino-1-(4-morpholinophenyl)butanone-1,2-methyl-1-[4-(methoxythio)-phenyl]-2-morpholinopropan-2-one, and the like. They may be usedsingly or in combination. Examples of the α-amino alkyl phenone compoundthat is available on the market include IRGACURE 369 and IRGACURE 907available from Ciba AG.

Specific examples of the thioxanthone compound include thioxanthone,2-methyl thioxanthone, 2-ethyl thioxanthone, 2-isopropyl thioxanthone,4-isopropyl thioxanthone, 2-chloro thioxanthone, 2,4-dimethylthioxanthone, 2,4-diethyl thioxanthone, 2,4-dichloro thioxanthone,1-chloro-4-propoxy thioxanthone, and the like. They may be used singlyor in combination. Examples of the thioxanthone compound that isavailable on the market include MKAYACURE DETX-S available from NipponKayaku, Co., Ltd. and ITX-S available from Double Bond Chemical Ind.,Co., Ltd.

The amount of the photopolymerization initiator in the ink compositionis, although it depends on the amount of the polymerizable compound,preferably 2 to 15 mass % in total with respect to the total mass of theink composition. When the amount of the photopolymerization initiator is2 mass % or more, it is possible to obtain an ink having excellentcurability and adhesion even with low-energy radiation. On the otherhand, when the amount of the photopolymerization initiator is 15 mass %or less, it is possible to suppress unreacted component residues.

Coloring Agent

The ink composition according to the present invention may furthercontain a coloring agent. However, in the case where the ink compositionaccording to the present invention is a clear ink composition that iscolorless and transparent, a coloring agent is not contained.

There is no particular limitation on the coloring agent. However, theink composition according to the present invention is non-aqueous, andit is therefore preferable to use a pigment that is easily dispersed ina uniform manner in a non-aqueous medium, or a dye that is easilydissolved in a non-aqueous medium.

The pigment may be an inorganic pigment or an organic pigment. Examplesof the inorganic pigment include titanium oxide, zinc white, zinc oxide,lithophone, iron oxide, aluminum oxide, silicon dioxide, kaolinite,montmorillonite, talc, barium sulfate, calcium carbonate, silica,alumina, cadmium red, red oxide, molybdenum red, chrome vermilion,molybdate orange, yellow lead chromate, chrome yellow, cadmium yellow,yellow iron oxide, titanium yellow, chromium oxide, viridian, cobaltgreen, titanium cobalt green, cobalt chromite green, lapis, ultramarineblue, deep blue, cobalt blue, cerulean blue, manganese violet, cobaltviolet, mica, and the like. As the organic pigment, examples includeorganic pigments such as azo pigments, azomethine pigments, polyazopigments, phthalocyanine pigments, quinacridone pigments, anthraquinonepigments, indigo pigments, thioindigo pigments, quinophthalone pigments,benzimidazolone pigments, isoindoline pigments, and the like. It is alsopossible to use carbon black made of acidic, neutral or basic carbon.Furthermore, it is also possible to use hollow particles of cross-linkedacrylic resin as the organic pigment.

Normally, in the ink composition according to the present invention, ablack pigment and three primary color pigments, namely, a cyan pigment,a magenta pigment, and a yellow pigment, are used, but it is alsopossible to use other color pigments, metallic luster pigments such as agold pigment and a silver pigment, colorless or pale color pigments, andthe like according to the intended application.

The coloring agents listed above may be used singly or in a combinationof two or more. Also, in the present invention, it is also possible touse two or more organic pigments or a solid solution of the organicpigment. It is also possible to use different coloring agents fordifferent droplets or liquids dropped, or use the same coloring agent.

For dispersion of the coloring agent, it is possible to use a dispersionapparatus such as, for example, a bead mill, a ball mill, a sand mill,an attritor mill, a roll mill, a jet mill, a homogenizer, a paintshaker, a kneader, an agitator, a Henschel mixer, a colloid mill, anultrasonic homogenizer, a pearl mill, and a wet jet mill. It is alsopossible to use a mixer such as a line mixer. Furthermore, afterdispersion of the coloring agent, for the purpose of removing coarseparticles of the coloring agent, classification processing may beperformed by using a centrifugal separator, a filter, or a crossflow.

When dispersing the coloring agent, a dispersant agent may be added.There is no particular limitation on the type of dispersant agent, butit is preferable to use a known polymer dispersant agent.

The amount of the dispersant agent can be selected as appropriateaccording to the intended use, and the amount of the dispersant agentcan be set to, for example, 0.01 to 5 mass % with respect to the totalmass of the ink composition.

When adding the coloring agent, it is also possible to use, as adispersion aid, a synergist according to the coloring agent wherenecessary.

The amount of the coloring agent can be selected as appropriateaccording to the color and intended use. From the viewpoint of imagedensity and storage stability, the amount of the coloring agent ispreferably 0.3 to 30 mass %, and more preferably 0.5 to 20 mass % withrespect to the total mass of the ink composition.

Other Components

It is preferable to add a polymerization inhibitor, an anti-gellingagent, and a surface controlling agent to the ink composition accordingto the present invention. By adding the polymerization inhibitor, andthe anti-gelling agent, the storage stability of the ink composition canbe improved. Also, by adding the surface controlling agent, the levelingproperty of printed matter surface can be improved.

As the polymerization inhibitor, for example, a hindered amine compound,a nitrosamine compound, a quinone compound or the like can be used. Asthe anti-gelling agent, for example, a hindered amine-based compound orthe like can be used. Also, as the surface controlling agent, forexample, polysiloxane or the like can be used.

Also, the ink composition according to the present invention maycontain, where necessary, additives such as an antifoaming agent, asanitizing agent, a moisturizing agent, a pH controlling agent, anantiseptic agent, and an antirust agent.

Next, a process for producing an ink composition according to thepresent invention will be described.

A first process for producing an ink composition according to thepresent invention includes: a step (A) of preparing an ink compositionprecursor containing a polymerizable compound and a photopolymerizationinitiator; a step (B1) of adding an alkali metal salt to the inkcomposition precursor and stirring the ink composition precursor at atemperature of 40° C. or more and 70° C. or less for a period of 10minutes or more and 120 minutes or less; a step (C) of holding and agingthe stirred ink composition precursor at a temperature of −20° C. ormore and 35° C. or less; and a step (D) of filtrating the aged inkcomposition precursor to produce an ink composition.

Also, a second process for producing an ink composition according to thepresent invention includes: a step (A) of preparing an ink compositionprecursor containing a polymerizable compound and a photopolymerizationinitiator; a step (B2) of stirring the ink composition precursor at atemperature of 40° C. or more and 70° C. or less for a period of 10minutes or more and 120 minutes or less; a step (C) of holding and agingthe stirred ink composition precursor at a temperature of −20° C. ormore and 35° C. or less; and a step (D) of filtrating the aged inkcomposition precursor to produce an ink composition, and at least one ofthe polymerizable compound and the photopolymerization initiatorcontains alkali metal ions.

Furthermore, the first and second processes for producing an inkcomposition according to the present invention may include a dewateringstep (E).

In the step (A), an ink composition precursor can be prepared byuniformly mixing a polymerizable compound and a photopolymerizationinitiator, and optionally a coloring agent, a polymerization inhibitor,an anti-gelling agent, a surface controlling agent, and other additivesthat were described above by using a stirrer. As the stirrer, forexample, a three-one motor stirrer, a magnetic stirrer, a diaper, ahomogenizer, or the like can be used.

In the step (B1) an alkali metal salt is added to the ink compositionprecursor so as to cause organic sulfonic acid that is contained in thepolymerizable compound to react with alkali metal ions. As the alkalimetal salt, it is possible to use, for example, organic acid alkalimetal salts such as sodium acetate, sodium tartrate, sodium citrate,sodium benzoate, and sodium acrylate; and inorganic acid alkali metalsalts such as sodium chloride, sodium carbonate, sodiumhydrogencarbonate, and sodium hydroxide; and the like, but it ispreferable to use an alkali metal salt, such as sodium acrylate, thateasily reacts with the organic sulfonic acid and is easily dissolved inthe polymerizable compound according to the HSAB concept.

There is no particular limitation on the addition amount of the alkalimetal salt, and the addition amount of the alkali metal salt ispreferably 0.001 mass % or more and 0.1 mass % or less with respect tothe total mass of the ink composition precursor. When the additionamount of the alkali metal salt is 0.001 mass % or more, the alkalimetal salt sufficiently reacts with the organic sulfonic acid containedin the ink composition precursor, and it is possible to suppress theformation of precipitates while the ink composition is stored. On theother hand, when the addition amount of the alkali metal salt is 0.1mass % or less, it is also possible to suppress the formation ofprecipitates of the alkali metal salt itself while the ink compositionis stored.

In the step (B1), in the case where an alkali metal salt that is lesssoluble in the ink composition precursor is used, a step of dissolvingthe alkali metal salt in water and adding the alkali metal salt to theink composition precursor may be performed. In this case, it ispreferable to perform a dewatering step (E) in order to set the amountof water to 0.50 mass % or less with respect to the total mass of theink composition.

In the first process for producing an ink composition according to thepresent invention, it is necessary to add an alkali metal salt to theink composition precursor in the step (B1). However, the components ofthe ink composition such as the polymerizable compound and thephotopolymerization initiator may contain, for example, alkali metalions as impurities. In this case, it is unnecessary to further add analkali metal salt to ink composition precursor as in the step (B2) ofthe second process for producing an ink composition according to thepresent invention.

Whether the components of the ink composition contain, by nature, alkalimetal ions can be checked through analysis using an ICP emissionspectroscopic analyzer, an ICP mass spectrometer, an atomic absorptionspectrometer, or the like.

It is sufficient that the alkali metal ions are contained, in an amountof 5 to 200 ppm, in at least one of the components of the inkcomposition such as the polymerizable compound and thephotopolymerization initiator. Normally the alkali metal ions arecontained as impurities in the components such as the polymerizablecompound and the photopolymerization initiator, and the polymerizablecompound and the photopolymerization initiator do not contain alkalimetal ions as a basic component. Examples of the alkali metal ionsinclude sodium ions and potassium ions.

In the step (B2), the organic sulfonic acid contained in thepolymerizable compound is reacted with the alkali metal ions containedin the polymerizable compound, the photopolymerization initiator or thelike.

In the step (C), a reaction product between the organic sulfonic acidand the alkali metal ions is precipitated sufficiently. The agingtemperature is set to a temperature of preferably −20° C. or more and35° C. or less, and more preferably −20° C. or more and 10° C. or lessbecause a low aging temperature causes a supersaturated precipitationphenomenon. The low temperature aging period is set according to thefrequency of occurrence of the reaction product with respect to theorganic sulfonic acid, and is preferably set to a period of 60 minutesor more and 30 days or less. Furthermore, for the purpose ofaccelerating the reaction, it is possible to perform high temperatureaging at a temperature of 40° C. or more and 70° C. or less prior to lowtemperature aging. The high temperature aging period is preferably setto a period of 120 minutes or more and 20 days or less.

In the step (D), the ink composition precursor is filtrated by using afilter or the like so as to remove precipitates and produce an inkcomposition in which the amount of organic sulfonic acid is reduced.

The step (E) can be performed alone separate from the steps (A) to (D),but it is also possible to perform the step (E) in combination with thesteps (A) to (D). For example, in the case where the step (E) iscombined with the step (B1) or (B2), the step (E) can be performed as adewatering step by heat stirring that utilizes a difference inevaporation rate between the polymerizable compound and water. In thecase where the step (E) is combined with the step (D), a water absorbingstep that uses a water absorbing filler such as a molecular sieve, asilica gel, an activated alumina or an ion exchange resin can beperformed before or after the filtration step.

Through the steps (A) to (D), the amount of organic sulfonic acid can beset to 50 ppm or less when the ink composition according to the presentinvention is subjected to measurement that uses the aforementioned waterextraction method.

Also, because the steps (A) to (E) do not include a step of addingwater, or include a dewatering step, the amount of water can be set to0.50 mass % or less with respect to the total mass of the inkcomposition when the ink composition according to the present inventionis subjected to measurement that uses the aforementioned Karl Fischermethod.

Embodiment 2

Next, an ink-jet ink set according to the present invention will bedescribed. The ink-jet ink set according to the present inventionincludes the above-described ink composition according to the presentinvention. To be specific, the ink jet ink set according to the presentinvention includes a plurality of inks each composed of the inkcomposition according to the present invention. Examples of the inksinclude: pigment inks such as a black ink containing a black pigment, acyan ink containing a cyan pigment, a magenta ink containing a magentapigment, and a yellow ink containing a yellow pigment; and colorless andtransparent clear inks that do not contain a pigment.

Embodiment 3

Next, an ink-jet printing system according to the present invention willbe described. The ink-jet printing system according to the presentinvention is an ink jet printing system that uses the above-describedink composition according to the present invention and an ink-jetrecording apparatus, and the ink-jet recording apparatus includes an inkheating portion and an ink filter. Specifically, the ink heating portionis provided to control the viscosity of the ink composition to anoptimal discharge viscosity at the ink-jet head, and the ink filter isused to remove dirt, dust and extraneous matter introduced duringproduction process such as ink composition production and cartridgefilling and prevent the occurrence of clogging at an ink flow path and anozzle tip in the ink-jet head.

In the ink-jet printing system according to the present invention, anink-jet recording apparatus including an ink heating portion and an inkfilter is used, and thus if an ink composition is used in which theamount of organic sulfonic acid is above 50 ppm and the amount of wateris above 0.50 mass %, the formation of reaction product is accelerateddue to the ink in a heated state to produce a large amount ofprecipitates, which clogs the entire ink filter and blocks the ink flowpath. Accordingly, the ink composition does not reach the nozzle tip.If, on the other hand, the ink composition according to the presentinvention is used in which the amount of organic sulfonic acid is 50 ppmor less and the amount of water is 0.50 mass % or less, even when theink is heated to reduce the ink viscosity, the formation of reactionproduct is suppressed, and thus the ink flow path or the nozzle tip inthe ink-jet head is not clogged by precipitates.

There is no particular limitation on the ink-jet technology used in theink-jet recording apparatus, but it is possible to use electric chargecontrol technology in which the ink is discharged by utilizing a staticattraction force, drop-on-demand technology (pressure pulse technology)that utilizes vibration pressure of a piezoelectric element, acousticink-jet technology that utilizes radiation pressure applied to the inkby changing an electric signal into an acoustic beam, and thermalink-jet technology that utilizes a pressure generated by heating the inkto form air bubbles.

In the ink-jet printing system according to the present invention, forexample, an ink is discharged from an ink-jet ink set that includes theabove-described ink composition of the present invention by using anink-jet printer, and thereafter energy rays are applied to the ink,whereby the ink can be fixed. As the energy rays, it is possible to use,for example, ultraviolet rays having a wavelength of 200 to 400 nm,far-ultraviolet rays, g rays, h rays, i rays, KrF excimer laser light,ArF excimer laser light, electron beams, X rays, molecular beams, LEDrays, and the like.

EXAMPLES

Hereinafter, the present invention will be described by way of examples,but it is to be understood that the present invention is not limited tothe examples given below.

Table 1 shows components that were used to prepare inks in examples andcomparative examples described below.

TABLE 1 Component Product Name Details/Product Name • SpecificationsColoring Black MA-8 Acidic carbon black pigment/available fromMitsubishi Chemical Corporation Agent Product name: MA-8 Cyan P-BFSCopper phthalocyanine pigment/available from Clariant Product name:HOSTAPERM BLUE P-BFS Magenta RT343D Quinacridone pigment/available fromCiba AG Product name: CINQUASIA Magenda RT-343-D Yellow LA-2 Nickle azopigment/available from Ciba AG Product name: LA-2 White CR-60 Titaniumoxide/available from Ishihara Sangyo Co., Ltd. Product name: TIPAQUECR-60 Dispersant Agent SOLSPERSE Comb-type copolymer having basicfunctional group/availible from Lubrizol 32000 Corporation Product name:SOLSPERSE 32000 Polymerizable Compound THF-A Tetrahydrofurfurylacrylate/available from Sartomer, Co. Product name: Sartomer SR285(ethylenic double bonds per molecule: 1) HDDA Hexanedioldiacrylate/available from Sartomer, Co. Product name: Sartomer SR238F(ethylenic double bonds per molecule: 2) PE-3A Pentaerythritoltriacrylate/available from Kyoeisha Chemical, Co., Ltd. Product name:Light Acrylate PE-3A (ethylenic double bonds per molecule: 3)Anime-Modified Polymerizable EBECRYL Amine-modified oligomer/availablefrom Daicel-Allnex, Ltd. Compound 7100 Product Name: EBECRYL 7100(ethylenic double bonds per molecule: 2) EBECRYL 80 Amine-modifiedoligomer/available from Daicel-Allnex, Ltd. Product name: EBECRYL 80(ethylenic double bonds per molecule: 4) Photopolymerization IRGACURE2-methyl-1-[4-(methyl thio)phenyl]-2-morpholinopropan-1-one/availablefrom Initiator 907 Ciba AG Product name: IRGACURE 907 (α-amino alkylphenone compound, sodium content 35 ppm) DAROCURE 2,4,6-trimethylbenzoyl diphenyl phosphine oxide/available TPO from Ciba AG Productname: DAROCUR TPO (acylphosphine oxide compound, sodium content 67 ppm)ITX-S Isopropyl thioxanthone/available from Double Bond Chemical Co.,Ltd. Product name: ITX-S (thioxanthone compound, sodium content 24 ppm)Surface Controlling BYK-3510 Polyether-modified polydimethylsiloxane/available from BYK-Chemie Agent Product name: BYK-3510Anti-Gelling UV-10 Bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-4-yl)sebacate/available from Ciba AG Agent Product name:IRGASTAB UV-10 Polymerization MQ Methoquinone/available from KawaguchiChemical Industry Co., Ltd. Inhibitor Product name: MQ

Next, the amount of organic sulfonic acid was measured by using a waterextraction method at a temperature of 25° C. with respect to each of thecomponents such as the polymerizable compounds, the amine-modifiedpolymerizable compounds, the photopolymerization initiators, the surfacecontrolling agent, the anti-gelling agent, and the polymerizationinhibitor listed above. To be specific, first, 3 parts by mass of eachcomponent was added to 25 parts by mass of ion exchanged water, whichwas then subjected to ultrasonic processing for 5 minutes and stirringprocessing using a mixer for 1 minute, and the mixture was filtrated byusing a hydrophilic filter having a pore size of 0.2 μm so as to producea measurement solution. Next, the amount of organic sulfonic acid in themeasurement solution was determined by using a liquid chromatographytime-of-flight mass spectrometer (LC/MS). The spectrometer had ameasurement threshold of 0.4 ppm or less. The results are shown in Table2.

TABLE 2 Component Product Name Amount of Organic Sulfonic Acid (ppm)Polymerizable Compound THF-A Below detection threshold HDDA Belowdetection threshold PE-3A 4 Amine-Modified EBECRYL 7100 87 PolymerizableCompound EBECRYL 80 318 Photopolymerization Initiator IRGACURE 907 Belowdetection threshold DAROCURE TPO Below detection threshold ITX-S Belowdetection threshold Surface Controlling Agent BYK-3510 Below detectionthreshold Anti-Gelling Agent UV-10 Below detection thresholdPolymerization Inhibitor MQ Below detection threshold

Examples 1 to 5 Preparation of Pigment Ink

First, a primary dispersion of coloring agent (pigment) was prepared inthe manner described below. Specifically, a coloring agent, a dispersantagent, and polymerizable compounds were weighed in the blending amounts(unit: part by mass) as shown in Table 3, and placed in a plasticbottle. Then, 100 parts by mass of zirconia beads having a diameter of0.3 mm was added thereto, and the obtained mixture was subjected todispersion processing using a paint conditioner for 1 hour.

Next, a pigment ink was prepared by using the primary dispersionobtained above. Specifically, the remaining components other thanphotopolymerization initiators were added to the primary dispersion inthe blending amounts (unit: part by mass) shown in Table 3, and theobtained mixture was stirred by using a magnetic stirrer for 30 minutes.After the mixture was stirred, the mixture was subjected to suctionfiltration by using a glass filter (available from Kiriyama Glass WorksCo.) so as to prepare a pigment ink precursor.

Then, photopolymerization initiators (compounds containing alkali metalion impurities) were added to the pigment ink precursor in the blendingamounts (unit: part by mass) shown in Table 3, which was then stirred ata temperature of 50° C. for 30 minutes. After that, the pigment inkprecursor was held at a temperature of 25° C. for 120 minutes, andthereafter filtrated by using a glass filter (available from KiriyamaGlass Works Co.). In this way, pigment inks of Examples 1 to 5 wereprepared.

TABLE 3 Product Example 1 Example 2 Example 3 Example 4 Example 5Component Name Black Ink Cyan Ink Magenta Ink Tellow Ink White InkColoring Black MA-8 2.00 — — — — Agent Cyan P-BFS — 2.00 — — — (Pigment)Magenta RT343D — — 2.00 — — Yellow LA-2 — — — 2.00 — White CR-60 — — — —10.00 Dispersant Agent SOLSPERSE 1.00 1.00 1.00 1.00 1.00 32000Polymerizable THF-A 20.00 20.00 20.00 20.00 20.00 Compound HDDA 33.6933.69 33.69 33.69 28.69 PE-3A 20.00 20.00 20.00 20.00 20.00Amine-Modified EBECRYL 10.00 10.00 10.00 10.00 10.00 Polymerizable 7100Compound EBECRYL 80 — — — — — Photopoly- IRGACURE 10.00 10.00 10.0010.00 — merization 907 Initiator DAROCURE — — — — 10.00 TPO ITX-S 3.003.00 3.00 3.00 — Surface Controlling BYK-3510 0.01 0.01 0.01 0.01 0.01Agent Anti-Gelling Agent UV-10 0.30 0.30 0.30 0.30 0.30 PolymerizationMQ — — — — — Inhibitor Total 100.00 100.00 100.00 100.00 100.00

Examples 6 and 7 Preparation of Clear Ink

The components other than photopolymerization initiators were weighed inthe blending amounts shown in Table 4 (unit: part by mass) and placed ina plastic bottle, which was then stirred by using a magnetic stirrer for30 minutes. After the mixture was stirred, the mixture was subjected tosuction filtration by using a glass filter (available from KiriyamaGlass Works Co.) so as to prepare a clear ink precursor.

Next, photopolymerization initiators (compounds containing alkali metalion impurities) were added to the clear ink precursor in the blendingamount (unit: part by mass) shown in Table 4, which was then stirred ata temperature of 50° C. for 30 minutes. After that, the clear inkprecursor was held at a temperature of 25° C. for 120 minutes, andthereafter filtrated by using a glass filter (available from KiriyamaGlass Works Ca). In this way, clear inks of Examples 6 and 7 wereprepared.

Example 8 Preparation of Clear Ink

The components other than photopolymerization initiators were weighed inthe blending amounts shown in Table 4 (unit: part by mass) and placed ina plastic bottle, which was then stirred by using a magnetic stirrer for30 minutes. After the mixture was stirred, the mixture was subjected tosuction filtration by using a glass filter (available from KiriyamaGlass Works Co.) so as to prepare a clear ink precursor.

Next, photopolymerization initiators (compounds containing alkali metalion impurities) were added to the clear ink precursor in the blendingamounts (unit: part by mass) shown in Table 4, which was then stirred ata temperature of 50° C. for 30 minutes. After that, the clear inkprecursor was held at a temperature of 25° C. for 120 minutes, thensubjected to a dewatering step by causing the clear ink precursor topass through a column filled with molecular sieve (4A, which is aproduct available from Nacalai Tesque, Inc.), and thereafter filtratedby using a glass filter (available from Kiriyama Glass Works Co.). Inthis way, a clear ink of Example 8 was prepared.

Example 9 Preparation of Clear Ink

The components were weighed in the blending amounts shown in Table 4(unit: part by mass) and placed in a plastic bottle, which was thenstirred by using a magnetic stirrer for 30 minutes. After the mixturewas stirred, the mixture was subjected to suction filtration by using aglass filter (available from Kiriyama Glass Works Co.) so as to preparea clear ink precursor.

Next, 0.02 parts by mass of sodium acrylate (alkali metal salt) wasadded to 100 parts by mass of the clear ink precursor, which was thenstirred at a temperature of 60° C. for 30 minutes. After that, the clearink precursor was held at a temperature of −10° C. for 24 hours, andthereafter filtrated by using a glass filter (available from KiriyamaGlass Works Co.). In this way, a clear ink of Example 9 was prepared.

Example 10 Preparation of Clear Ink

The components other than photopolymerization initiators were weighed inthe blending amounts shown in Table 4 (unit: part by mass) and placed ina plastic bottle, which was then stirred by using a magnetic stirrer for30 minutes. After the mixture was stirred, the mixture was subjected tosuction filtration by using a glass filter (available from KiriyamaGlass Works Co.) so as to prepare a clear ink precursor.

Next, photopolymerization initiators (compounds containing alkali metalion impurities) were added to the clear ink precursor in the blendingamounts (unit: part by mass) shown in. Table 4, which was then stirredat a temperature of 60° C. for 60 minutes. After that, the clear inkprecursor was held at a temperature of 60° C. for 7 days, and then at atemperature of −10° C. for 7 days, then subjected to a dewatering stepby causing the clear ink precursor to pass through a column filled withmolecular sieve (4A, which is a product available from Nacalai Tesque,Inc.), and thereafter filtrated by using a glass filter (available fromKiriyama Glass Works Co.). In this way, a clear ink of Example 10 wasprepared.

TABLE 4 Product Example 6 Example 7 Example 8 Example 9 Example 10Component Name Clear Ink Clear Ink Clear Ink Clear Ink Clear InkPolymerizable THF-A 20.00 20.00 20.00 20.00 20.00 Compound HDDA 39.6942.69 42.67 39.67 39.67 PE-3A 20.00 20.00 20.00 20.00 20.00Amine-Modified EBECRYL 10.00 — — 10.00 10.00 Polymerizable 7100 CompoundEBECRYL 80 — 7.00 7.00 — — Photopoly- IRGACURE — — — — — merization 907Initiator DAROCURE 10.00 10.00 10.00 10.00 10.00 TPO ITX-S — — — — —Surface Controlling BYK-3510 0.01 0.01 0.01 0.01 0.01 Agent Anti-GellingAgent UV-10 0.30 0.30 0.30 0.30 0.30 Polymerization MQ — — 0.02 0.020.02 Inhibitor Total 100.00 100.00 100.00 100.00 100.00

Comparative Examples 1 and 2 Preparation of Pigment Ink

First, a primary dispersion of coloring agent (pigment) was prepared inthe manner described below. Specifically, a coloring agent, a dispersantagent, and polymerizable compounds were weighed in the blending amounts(unit: part by mass) as shown in Table 5, and placed in a plasticbottle. Then, 100 parts by mass of zirconia beads having a diameter of0.3 mm was added thereto, and the obtained mixture was subjected todispersion processing using a paint conditioner for 1 hour.

Next, a pigment ink was prepared by using the primary dispersionobtained above. Specifically, the remaining components other thanphotopolymerization initiators were added to the primary dispersion inthe blending amounts (unit: part by mass) shown in Table 5, and theobtained mixture was stirred by using a magnetic stirrer for 30 minutes.After the mixture was stirred, the mixture was subjected to suctionfiltration by using a glass filter (available from Kiriyama Glass WorksCo.) so as to prepare a pigment ink precursor.

Then, photopolymerization initiators (compounds containing alkali metalion impurities) were added to the pigment ink precursor in the blendingamounts (unit: part by mass) shown in Table 5, which was then stirred ata temperature of 50° C. for 30 minutes. After that, the pigment inkprecursor was held at a temperature of 25° C. for 120 minutes, andthereafter filtrated by using a glass filter (available from KiriyamaGlass Works Co.). In this way, pigment inks of Comparative Examples 1and 2 were prepared.

Comparative Examples 3 and 4 Preparation of Clear Ink

The components other than photopolymerization initiators were weighed inthe blending amounts shown in Table 5 (unit: part by mass) and placed ina plastic bottle, which was then stirred by using a magnetic stirrer for30 minutes. After the mixture was stirred, the mixture was subjected tosuction filtration by using a glass filter (available from KiriyamaGlass Works Co.) so as to prepare a clear ink precursor.

Next, photopolymerization initiators (compounds containing alkali metalion impurities) were added to the clear ink precursor in the blendingamounts (unit: part by mass) shown in Table 5, which was then stirred ata temperature of 50° C. for 30 minutes. After that, the clear inkprecursor was held at a temperature of 25° C. for 120 minutes, andthereafter filtrated by using a glass filter (available from KiriyamaGlass Works Co.). In this way, clear inks of Comparative Examples 3 and4 were prepared.

TABLE 5 Comparative Comparative Comparative Comparative Product Example1 Example 2 Example 3 Example 4 Component Name Black Ink White Ink ClearInk Clear Ink Coloring Back MA-8 2.00 — — — Agent White CR-60 — 10.00 —— (Pigment) Dispersant Agent SOLSPERSE 1.00 1.00 — — 32000 PolymerizableTHF-A 20.00 20.00 20.000 20.00 HDDA 33.69 28.69 39.690 42.69 PE-3A 20.0020.00 20.000 20.00 Amine-Modified EBECRYL 10.00 10.00 10.000 —Polymerizable 7100 Compound EBECRYL 80 — — — 7.00 Photopoly- IRGACURE10.00 — — — merization 907 Initiator DAROCURE — 10.00 10.000 10.00 TPOITX-S 3.00 — — — Surface Controlling BYK-3510 0.01 0.01 0.010 0.01 AgentAnti-Gelling Agent UV-10 0.30 0.30 0.300 0.30 Polymerization MQ — — — —Inhibitor Organic Methanesulfonic 0.01 — 0.005 — Sulfonic Acid AcidWater Ion Exchanged — 1.00 — 0.50 Water Total 100.01 101.00 100.005100.50

Next, the amount of organic sulfonic acid and the amount of water weremeasured in the manner described below by using the pigment inks andclear inks prepared in the manner described above in Examples 1 to 10and Comparative Examples 1 to 4 immediately after preparation thereof.The results are shown in Table 6.

Amount of Organic Sulfonic Acid

A measurement solution was prepared by adding 3 parts by mass of eachink to 25 parts by mass of ion exchanged water, which was then subjectedto ultrasonic processing for 5 minutes and stirring processing using amixer for 1 minute, and thereafter filtrated by using a hydrophilicfilter having a pore size of 0.2 μm. Next, the amount of organicsulfonic acid in the measurement solution was determined by using aliquid chromatography time-of-flight mass spectrometer (LC/MS). Thespectrometer had a measurement threshold of 0.4 ppm or less. As usedherein, the amount of organic sulfonic acid refers to a value obtainedby performing three instances of measurement by using the samemeasurement solution and arithmetically averaging the results of thethree instances of measurement.

Amount of Water

The amount of water in each ink was measured by using a Karl Fischermethod. To be specific, the amount of water was measured by using a KarlFischer moisture analyzer system (equipped with a Karl Fischer moistureanalyzer (volumetric titration method) MKC-610 and a moisture evaporatorADP-351) available from Kyoto Electronics Manufacturing Co., Ltd.

Subsequently, ink characteristics were evaluated in the manner describedbelow by using the pigment inks and clear inks prepared in the mannerdescribed above in Examples 1 to 10 and Comparative Examples 1 to 4immediately after preparation thereof. The results are shown in Table 6.

Storage Stability

Each ink was charged in a glass bottle, which was then subjected to onecycle of heating/cooling test in which the ink was stored at 60° C. for2 days and then at −10° C. for 2 days by using an environmental testchamber. After that, the ink was subjected to suction filtration byusing a SUS mesh (with a pore size of 5 μm), and the state of residue onthe mesh was observed by using an optical microscope so as to determinethe presence or absence of the formation of precipitates in the ink.Then, the storage stability of the ink was evaluated based on thefollowing criteria.

Rating A: No residue was observed.

Rating B: A slight amount of residue was observed

Rating C: Residue was observed

Rating D: A large amount of residue was observed

Ink-jet (IJ) Dischargeability

With respect to each ink, the IJ dischargeability of the ink after thestorage stability test was evaluated by using an ink-jet recordingapparatus equipped with a piezo-type ink-jet nozzle. The ink-jetrecording apparatus had, as an ink supply system, an ink tank, a supplypipe, an upstream ink tank provided immediately ahead of the head, and apiezo head. Also, the upstream ink tank provided immediately ahead ofthe head and the piezo head were provided with a filter (a SUS mesh witha pore size of 5 μm) for removing precipitates in the ink. Furthermore,when discharging the ink, the ink was heated by a temperature controlsystem provided in the ink-jet recording apparatus such that the ink hadan optimal discharge viscosity at the head of 8 to 13 mPa·s. Also, theink-jet recording apparatus was driven at a drive frequency of 10 kHz soas to discharge the ink with a droplet size of about 7 pl and aresolution of 600×600 dpi. At this time, the IJ dischargeability of theink was evaluated based on the following criteria.

Rating A: A loss of ink droplets was not observed at all.

Rating B: A slight loss of ink droplets was observed.

Rating C: A loss of ink droplets was observed.

Rating D: A great loss of ink droplets was observed.

Curability

Each ink was applied to a 188 μm thick polyethylene terephthalate film(U292W, which is a white PET film available from DuPont Thijin Films)with the use of a bar coater so as to form a 3 μm thick ink solid printfilm. The print film was irradiated with ultraviolet rays by using, aslight application means, an ultraviolet LED lamp (NLBU21W01-E2, which isa product available from Nichia Corporation, with a peak luminousintensity of 38.7 mW/cm²) until a total amount of irradiation lightreached 200 mJ/cm², so as to cure the ink, and a cured solid printedmatter was obtained. The curability of the ink was evaluated based onthe following criteria by touching the cured product with a finger.

Rating A: The ink did not adhere to the finger at all.

Rating B: The ink slightly adhered to the finger.

Rating C: The ink adhered to the finger.

Rating D: The ink was uncured.

TABLE 6 Amount of Amount Organic of IJ Type Sulfonic Acid Water StorageDischarge- of Ink (ppm) (mass %) Stability ability Curability Example 1Black Ink 14 0.15 A A B Example 2 Cyan Ink 14 0.14 A A B Example 3Magenta Ink 15 0.16 A A B Example 4 Yellow Ink 14 0.12 A A A Example 5White Ink 16 0.13 A A B Example 6 Clear Ink 16 0.15 B A A Example 7Clear Ink 29 0.15 B B B Example 8 Clear Ink 29 0.07 A A B Example 9Clear Ink 7 0.15 A A A Example 10 Clear Ink 16 0.03 A A A ComparativeBlack Ink 114 0.15 D D C Example 1 Comparative White Ink 16 1.13 C C DExample 2 Comparative Clear Ink 66 0.15 D D D Example 3 ComparativeClear Ink 29 0.65 D D D Example 4

From Table 6, it can be seen that the inks of Examples 1 to 10 exhibitedsatisfactory results in all of storage stability, IJ dischargeability,and curability On the other hand, in Comparative Examples 1 and 3 inwhich the amount of organic sulfonic acid was above 50 ppm and inComparative Examples 2 and 4 in which the amount of water was above 0.50mass %, it was not possible to obtain satisfactory results in all ofstorage stability, IJ dischargeability, and curability.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide an inkcomposition that has excellent storage stability IJ dischargeability andcurability; and an inkjet ink set including the ink composition.

In addition, the present invention is applicable not only to a processfor producing an ink for use in printing that uses ink-jet technologyand a formed matter, and a printing system that uses inkjet technology,for example, ordinary flatbed printing that uses ink-jet technology, butalso to printing that uses ink-jet technology and provides athree-dimensional finish by stacking up ink, or a three-dimensionalshaping process (ink-jet stereolithographic process or the like) by inkjet technology.

1. An ink composition comprising a polymerizable compound and aphotopolymerization initiator, wherein an amount of organic sulfonicacid measured at a temperature of 25° C. by using a water extractionmethod is 50 ppm or less, and an amount of water measured by a KarlFischer method is 0.50 mass % or less with respect to a total mass ofthe ink composition.
 2. The ink composition according to claim 1,wherein the water extraction method is a method in which the amount oforganic sulfonic acid in a measurement solution produced by mixing theink composition with water is measured by chromatography.
 3. The inkcomposition according to claim 1, wherein the organic sulfonic acid isat least one selected from the group consisting of methane sulfonicacid, ethane sulfonic acid, trifluoromethane sulfonic acid, benzenesulfonic acid, dodecylbenzene sulfonic acid, toluene sulfonic acid, andnaphthalene sulfonic acid.
 4. The ink composition according to claim 1,wherein the polymerizable compound includes an amine-modifiedpolymerizable compound.
 5. The ink composition according to claim 1,further comprising a coloring agent.
 6. A process for producing an inkcomposition, the process comprising the steps of: preparing an inkcomposition precursor containing a polymerizable compound and aphotopolymerization initiator; adding an alkali metal salt to the inkcomposition precursor and stirring the ink composition precursor at atemperature of 40° C. or more and 70° C. or less for a period of 10minutes or more and 120 minutes or less; holding and aging the stirredink composition precursor at a temperature of −20° C. or more and 35° C.or less; and filtrating the aged ink composition precursor to produce anink composition.
 7. A process for producing an ink composition, theprocess comprising the steps of: preparing an ink composition precursorcontaining a polymerizable compound and a photopolymerization initiator;stirring the ink composition precursor at a temperature of 40° C. ormore and 70° C. or less for a period of 10 minutes or more and 120minutes or less; holding and aging the stirred ink composition precursorat a temperature of −20° C. or more and 35° C. or less; and filtratingthe aged ink composition precursor to produce an ink composition,wherein at least one of the polymerizable compound and thephotopolymerization initiator contains alkali metal ions.
 8. The processfor producing an ink composition according to claim 6, furthercomprising a dewatering step.
 9. The process for producing an inkcomposition according to claim 6, wherein the polymerizable compoundcontains an amine-modified polymerizable compound.
 10. The process forproducing an ink composition according to claim 6, wherein the inkcomposition precursor further contains a coloring agent.
 11. An ink-jetink set comprising the ink composition according to claim
 1. 12. Anink-jet printing system that uses the ink composition according to claim1 and an ink-jet recording apparatus, wherein the ink-jet recordingapparatus includes an ink heating portion and an ink filter.
 13. Theprocess for producing an ink composition according to claim 7, furthercomprising a dewatering step.
 14. The process for producing an inkcomposition according to claim 7, wherein the polymerizable compoundcontains an amine-modified polymerizable compound.
 15. The process forproducing an ink composition according to claim 7, wherein the inkcomposition precursor further contains a coloring agent.